1. Field of the Invention
The present invention relates to an input circuit and an output circuit for a semiconductor integrated circuit.
2. Description of the Related Art
Generally, an electronic apparatus including a plurality of semiconductor integrated circuits has a large capacitance provided by a plurality of capacitors and a large parasitic capacitance in the power line which supplies electrical power to the semiconductor integrated circuits. As a result, the power source voltage VBAT gradually rises. On the other hand, each semiconductor integrated circuit has a range of power source voltage VBAT so that a desired function or action can properly be achieved. Therefore, in order to stop the function of the semiconductor integrated circuits under a specified voltage, and to allow for the performance of the function of the semiconductor integrated circuits over the specified voltage, a UVLO (Under Voltage Lock Out) circuit is externally or internally provided and arranged to detect the specified voltage of power source voltage VBAT (See, for example, Japanese Patent Application Laid-Open No.2001-296930). Therefore, while the power source voltage VBAT rises, the semiconductor integrated circuit is prevented from malfunctioning by stopping the function thereof until the power source voltage VBAT reaches a voltage at which proper functioning is possible.
FIG. 3 shows an example of a conventional semiconductor integrated circuit with an internal UVLO circuit. The semiconductor integrated circuit 101 includes an input circuit 102 which inputs a signal from an input terminal IN, a function circuit 103 which actually performs the function of the semiconductor integrated circuit 101, and a UVLO circuit 104. The input circuit 102 is a circuit which inputs either a high-level or low level from the input terminal IN, and outputs a same polarity level as the input level. The input circuit 102 includes a N-type MOS transistor 111, the gate of which a signal from the input terminal IN is input to and the source of which is grounded, a resistor 116 arranged between the drain of the transistor 111 and the power source voltage VBAT, and an inverter 119 which inputs a signal from the node between the drain of the transistor 111 and the resistor 116 and outputs a high-level or low level. The UVLO circuit 104 is a circuit which detects the specified voltage of the power source voltage VBAT and outputs a high level or low level. The UVLO circuit 104 includes resistors 131 and 132 which divide the power source voltage VBAT, a reference voltage generating circuit 133 which generates a reference voltage VREF, and a comparator 134 which compares the divided voltage of the power source voltage VBAT and the reference voltage VREF and outputs a high-level or low level. The output of the inverter 119 and the output of the comparator 134 are input to an AND circuit 135, and the output of the AND circuit 135 is input to the function circuit 103. The input voltage for the function circuit 103 will be fixed at a low level in order to stop (disable) the operation of the function circuit 103.
When the divided voltage of power source voltage VBAT is lower than the reference voltage VREF, the UVLO circuit 104 determines that the power source voltage VBAT is not at a voltage capable for proper functioning and outputs a low level. Therefore, in this case, since the AND circuit 135 will output a low level, the function circuit 103 will not function.
Hence, the UVLO circuit 104 forces the function circuit 103 not to operate if the power source voltage VBAT is below a specified voltage where proper functioning is possible, so even if the power source voltage VBAT rises gradually, malfunctioning can be prevented. However, the UVLO circuit 104 is constantly comparing the power source voltage VBAT and the reference voltage VREF, so a relatively large DC current is always flowing to the resistors 131 and 132, the reference voltage generating circuit 133, and the comparator 134. In other words, this DC current is flowing not only while the power source voltage VBAT rises, but even after the voltage rise is complete, so a significant amount of electrical power is consumed. Furthermore, the circuit volume of the UVLO circuit 104 is large. If the circuit volume of the function circuit 103 of the semiconductor integrated circuit 101 is small, achieving this type of internal UVLO circuit 104 will be actually difficult. On the other hand, if an external UVLO circuit 104 is used, a terminal for inputting that signal will be necessary, and it will be necessary to attach wiring from the external UVLO circuit 104 on the printed board.